Recording materials used in a heat-sensitive recording process are usually acid-developable heat-sensitive recording materials. However, the recording materials of this type undergo unfavorable discoloration or color formation after recording due to severe handling or contact with adhesive tapes or diazo type copying papers.
Intensive studies on diazo-developable heat-sensitive recording materials have recently been conducted for developing recording materials free from the above-described disadvantages. However, since diazo compounds are inherently labile, the recording materials of this type are susceptible to background coloration under various conditions.
It has been proposed, on the other hand, to improve durability of recorded images by providing a protective layer on general heat-sensitive recording materials. However, formation of a protective layer or an anticurl backing layer on the coated surface or the back surface increases cost of production, though achieving the above-mentioned particular purpose. It has, therefore, been demanded to develop a heat-sensitive material which exhibits excellent durability and can be produced at low cost.
Japanese patent application No. 99490/84(corresponding to U.S. patent application Ser. No. 735,227) and Japanese patent application (OPI) No. 91438/84 (corresponding to U.S. Pat. No. 4,529,681) (the term "OPI" herein used means "unexamined published application") disclose heat-sensitive recording materials comprising a support carrying on the same surface thereof microcapsules containing a basic colorless dye (hereinafter referred to as a leuco dye), a monomer and/or other organic solvents as core materials and a developer which develops the leuco dye. Upon application of heat, a color forming component contained in the core of microcapsules is penetrated out of the capsules through the capsule wall, or an another component outside the capsules which is capable of color formation reaction penetrates into the capsules through capsule walls. In either case, color formation reaction takes place to develop a color.
In the case where microcapsules contain a photopolymerization initiator and a vinyl monomer as core materials as disclosed in Japanese patent application (OPI) No. 91438/84 (corresponding to U.S. Pat. No. 4,529,681), the entire surface of the recording materials after recording is exposed to light to polymerize the vinyl monomer present in the core whereby the color forming component can be prevented from permeation to prevent color development of the non-developed area (i.e., fixation).
Since the color formation mostly takes place within the capsules as confirmed by microscopic observation and the like, the reactants for the color formation are actually isolated from each other by the partitioning capsule walls either before or after the recording even if the above-described fixation is not carried out. Therefore, higher durability and stability than as obtained in the heat-sensitive recording materials having the aforesaid protective layer can be obtained.
Despite of the above-described outstanding characteristics, the heat-sensitive recording materials using microcapsules do not withstand use due to their low color density unless the oil for the core materials is properly selected.